1. Field of the Invention
The present invention relates to a valve control apparatus.
2. Description of Related Art
For instance, PCT publication WO2009/062928A1 (corresponding to U.S. 2010/0319663A1) teaches a valve control apparatus, which controls opening and closing of a valve. Specifically, with reference to FIGS. 17 and 18, the valve control apparatus includes a valve 101, an electric actuator and a motor control unit. The valve 101 is adapted to open and close a fluid passage. The electric actuator includes a rod 102, which drives the valve 101. The motor control unit controls electric power supplied to an electric motor 103, which is a drive power source of the electric actuator.
The electric actuator includes a speed reducing mechanism, a reciprocating slider link mechanism 300 and a thrust bearing 104. The speed reducing mechanism reduces a speed of rotation transmitted from the electric motor 103 through two stages. The reciprocating slider link mechanism converts rotational motion of the speed reducing mechanism into linear motion of the rod 102. The thrust bearing 104 slidably supports the rod 102 to enable reciprocation of the rod 102 in a reciprocating direction. The thrust bearing 104 includes a through-hole, which extends through the thrust bearing 104 in an axial direction of the rod 102, and the thrust bearing 104 is securely held in a bearing hole of an actuator case.
The speed reducing mechanism includes a pinion gear 105, an intermediate gear 106 and a final gear 107. The pinion gear 105 is fixed to an output shaft of the electric motor 103. The intermediate gear 106 is meshed with and is driven by the pinion gear 105. The final gear 107 is meshed with and is driven by the intermediate gear 106. The intermediate gear 106 is rotatably installed to an outer peripheral surface of a support shaft 111. The final gear 107 is rotatably installed to an outer peripheral surface of a support shaft 112.
A toggle lever 108 is connected to the rod 102 of the electric actuator through a first pivot 113. The toggle lever 108 is connected to the final gear 107 through a second pivot 114. The first pivot 113 is fitted into a first fitting hole of the toggle lever 108 and is thereby fixed to the toggle lever 108. The second pivot 114 is fitted into a second fitting hole of the toggle lever 108 and is thereby fixed to the toggle lever 108.
In the electric actuator of PCT publication WO2009/062928A1 (corresponding to US 2010/0319663A1), the electric motor 103 drives the gears 105-107 of the speed reducing mechanism, and the toggle lever 108, which is connected to the final gear 107 through the second pivot 114, pushes (or pulls) the rod 102 in an axial direction thereof to convert the rotational motion of the final gear 107 into the reciprocating linear motion of the rod 102.
Here, a link lever 109 is placed between the rod 102 and the shaft 115 of the valve 101.
In the electric actuator of PCT publication WO2009/062928A1 (corresponding to US 2010/0319663A1), when the link lever 109 is rotated by the rod thrust force, the rod 102 receives a reaction force of a side force (indicated by a dotted line in FIG. 17) from the link lever 109, so that swing motion is generated at a distal end part of the rod 102.
At the time of driving the rod 102, the rod 102 is driven to have linear motion in the rod axial direction Y and also curved motion in a curved direction Y′. Furthermore, a positional deviation occurs in a stroke position of the rod 102 due to a valve pressure P.
The sensor senses a location of a magnetic circuit (a magnetic circuit including a magnet and a yoke), which is provided in a rod position sensing member 110. Therefore, when the rod 102 is moved curvedly, the magnetic field from the magnet changes in a curved manner, and thereby an output value of the sensor changes in a curved manner. Thereby, a linearity of the output change characteristic of the sensor relative to the stroke position of the magnetic circuit, i.e., the stroke position of the rod 102 is deteriorated.
Furthermore, Japanese Unexamined Patent Publication No. 2004-177398A teachers a rod stroke position sensing apparatus, which can directly sense a linear stroke position of the rod 102. Specifically, as shown in FIG. 19A, the rod stroke sensing device includes a magnetic stationary body (two stators 121, 122 and a sensor held therebetween) and a magnetic circuit (a magnetic circuit having a permanent magnet 123 and a yoke 124). The magnetic circuit is displaceable relative to the magnetic stationary body in the stroke direction of the rod 102. The magnetic stationary body is placed parallel to the stroke direction (the axial direction) of the rod 102.
The sensor includes a Hall element 125, which is inserted into a magnetic flux sensing gap formed between the two stators 121, 122. The magnetic circuit is provided integrally to the rod 102.
The stators 121, 122, the permanent magnet 123, the yoke 124 and the Hall element 125 of the rod stroke position sensing apparatus form two magnetic circuits A, B.
In the rod stroke position sensing apparatus of Japanese Unexamined Patent Publication No. 2004-177398A, the magnetic circuits A, B are swung as indicated by arrows due to looseness of a rod bearing 133, which slidably supports the rod 102 to enable reciprocation of the rod 102 in the reciprocating direction. Thereby, a distance between the magnetic circuit and the Hall element 125 changes, so that the strength of the magnetic field, which is received by a magnetic sensing surface of the Hall element 125, varies (fluctuates).
Here, in a case where the magnetic circuit (the permanent magnet 123, the yoke 124) is placed close to the rod bearing 133, as shown in FIG. 19B, the amplitude of the swing of the magnetic circuit is small, and thereby a fluctuating range of the distance between the magnetic circuit and the Hall element 125 is also small. In contrast, in a case where the magnetic circuit is placed distant from the rod bearing 133, the amplitude of swing of the magnetic circuit becomes large, and the fluctuating range of the distance between the magnetic circuit and the Hall element 125 becomes large.
That is, the magnetic sensing surface of the Hall element 125 relative to the magnetic circuit is not placed in an average position of the amplitude of the magnetic circuit, so that there is a large difference in the amplitude of the swing of the magnetic circuit between the one case, in which the magnetic circuit is placed close to the rod bearing 133, and the other case, in which the magnetic circuit is placed distant from the rod bearing 133. Therefore, there is the fluctuation difference in the strength of the magnetic field, which is received by the magnetic sensing surface of the Hall element 125. As a result, there is the large difference in the sensor output in response to the swing of the rod 102, and thereby the sensor accuracy is deteriorated.